Recently, due to the exhaustion of fossil energy and environmental pollution, the interest in electric products which are capable of being driven by using electric energy without the use of fossil energy is growing more and more.
Accordingly, with the increasing development and demand of mobile device, electric vehicles, hybrid vehicles, power storage devices, uninterrupted power supplies, and the like, the demands of second batteries as energy sources are rapidly increasing, and also demand forms are being diversified.
Thus, more studies on secondary batteries are being actively carried out to cope with such diverse demands. In general, secondary batteries include nickel-cadmium batteries, nickel-hydrogen batteries, lithium ion batteries, and lithium ion polymer batteries.
Such secondary batteries are classified into lithium based batteries and nickel-hydrogen based batteries. Lithium-based batteries are mainly used for small products such as digital cameras, P-DVDs, MP3Ps, cellular phones, PDAs, portable game devices, power tools, and E-bikes, and nickel hydrogen-based batteries are mainly applied to and used for large products such as electric vehicles or hybrid electric vehicles, which need high output.
To drive electric vehicles or hybrid electric vehicles, a motor has to be driven, which requires high output. Also, in case of the power storage devices for supplying power to buildings or certain areas, a large amount of power that is enough to satisfy power demands has to be supplied. As described above, to provide power having high output or large capacity, a plurality of batteries, each of which is composed of a unit cell assembly, are connected in series or parallel to each other to supply desired output or power.
However, in case of the battery in which a plurality of battery cells are connected to each other, there may be made differences among charge capacities of the unit cells after the unit cells are repeatedly charged and discharged. If the discharging of the battery is continued in the state where the difference in charging capacity exists, it may be difficult to stably operate the battery because a specific unit cell having low charging capacity is overdischarged. On the other hand, if the charging of the battery is continued in the state where the difference in charging capacity exists, a specific unit cell may be overcharged to disturb the safety of the battery.
The difference in charging capacity may cause the overcharging or overdischarging of some of the unit cells. As a result, due to the above-described problem, power may not be stably supplied to a load (e.g., an electric motor, a power grid, and the like).
To solve this problem, various methods for continuously monitoring the charge capacity of the battery cells to balance the charge capacity of the battery cells to a constant level are required.